Flicker noise is a kind of noise caused by frequency mismatching when a subject is taken with an optical imaging device such as a camera in the lighting state of a fluorescent lamp adapted to be powered by an alternating current. Changes in black and white with time occur to an outputted image signal due to a difference between a luminance change frequency of the fluorescent lamp and a vertical synchronous frequency of the camera.
For example, in the case that a subject is taken with a CCD (Charge-Coupled Device) camera of an NTSC (National Television System Committee) mode (a vertical synchronous frequency is 60 Hz) in a light source of a fluorescent lamp of a non-inverter mode at a region where frequency of an alternating current is 50 Hz, because 1 cycle is 1/60 seconds but a luminance change cycle of the fluorescent lamp is 1/100 seconds, as changes in luminance of the fluorescent lamp make exposure timing of each field incorrect, which varies an exposure amount of each pixel.
In other words, a CMOS (Complementary Metal-Oxide-Semiconductor) image sensor used in an optical imaging device has a simple structure, is inexpensive, and allows high image quality and low power consumption. However, the CMOS image sensor has different timing for changing photoelectricity each pixel or line, and thus, in the case that a subject is taken in an environment where luminance changes cyclically, such as a fluorescent lamp, horizontal lines of black and white appears in an image. The horizontal lines are referred to as flicker or flicker noise.
Many methods for detecting flicker noise have been suggested, and mainly include a method for treating at a spatial area and a method for treating at a frequency area. The method for treating at a spatial area detects flicker noise components using a correlation with adjacent pixels, intervals between lines, or frames, and the method for treating at a frequency area detects flicker noise components using transform such as FFT (Fast Fourier Transform) or cepstrum.
The method for detecting flicker noise at a frequency area allows more accurate detection than the method for detecting flicker noise at a spatial area, however it requires too much hardware resources and memories to be incorporated into an embedded device or a chip.
On the other side, the method for detecting flicker noise at a spatial area using a correlation with adjacent pixels, intervals between lines, or frames has an increasing possibility of error in flicker noise detection according to noise components because of exposure to noise included in image information. Further, this method requires a memory for each line or frame to compare information of a current line with information of a next line.